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Effects of the Phase Locked Loop on the Stability of a Voltage Source Converter in a Weak Grid Environment

Korytowski, Matthew James
(2015)
Effects of the Phase Locked Loop on the Stability of a Voltage Source Converter in a Weak Grid Environment.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)

Abstract

Distributed generation is characterized as a form of generation that is not directly connected to the bulk transmission grid. It is usually connected via power electronic devices if it is a renewable resource, in this case in the form of a voltage source converter (VSC) operating as an inverter (converting DC-to-AC). The grid impedance connected to the VSC has an influence on its stability and control performance. By looking at the output impedance of the VSC, the stability can be determined in relation to the impedance of the grid connection. A number of parameters influence the output impedance of a VSC, one of those being the control scheme used and the phase-locked loop (PLL) contained within it. The control parameters of the PLL can be adjusted to manipulate the location of the calculated poles and zeros of the open loop output impedance of the VSC. These parameters are the gains of the loop filter, Kp and Ki. Under certain short circuit ratio (SCR) values, having large PLL gain parameters can cause the VSC to become unstable. While a large SCR will be unaffected by the PLL gains, a smaller SCR is more susceptible to PLL gains that are too large. By accounting for the effects of the PLL in the output impedance, it can be found what PLL gains are considered too large for certain SCR values. A large enough grid impedance can destabilize the VSC and, therefore, cause the renewable generation to be disconnected and unused. Ideally, the output impedance of the VSC will be large. This work analyzes the effect of the PLL on the output impedance of the VSC and ultimately the stability and control performance based on different grid impedances.